Fixes env handling to allow hard coding of environment variable in base job template

[desertaxle]

Updates propagation of env values into the job manifest generated by the Kubernetes worker to allow hard coding of env values in the base job template. With this change, users can add hardcoded environment variables to the base job template to enable the use of Kubernetes secrets like so:

"containers": [
    {
        "name": "prefect-job",
        "env": [
            {
                "name": "TEST_ENV",
                "valueFrom": {
                    "secretKeyRef": {
                        "name": "test-secret",
                        "key": "shhhhh",
                    }
                },
            },
        ],
        "image": "{{ image }}",
        "imagePullPolicy": "{{ image_pull_policy }}",
        "args": "{{ command }}",
    }
],

The Kubernetes worker will still handle adding the necessary Prefect setting environment variables to the job manifest, but those values will not override the user hardcoded values.

Closes #83
Closes PrefectHQ/prefect#10717

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